How fast is the Universe expanding?
Can you give me the exact velocity of the Universe to date please?
The parameter that is used to describe how fast the Universe is expanding is the Hubble constant (which tells us how fast the objects appear to be moving away from us as a function of distance).
Astronomers do not know for sure the value of the Hubble constant. In fact there are many debates going on about that, since it is a very important factor in understanding the history of the Universe. There are many estimates done with many different techniques, all of which have sizeable uncertainties associated with them. Generally, we assume the value to be between 50 and 100 km/s/Mpc (which means that for each Megaparsec (Mpc), galaxies appear to be receiding at 50-100 km/s, one Mpc being equivalent to 3.2 million light years)
The techniques used to estimate the Hubble constant involve looking at astronomical objects we understand the behaviour of, and comparing the observations at various distances from us. Some of the objects used are Cepheid variables (young and massive stars whose luminosity change periodically), supernovae (explosion of a star at the end of its life) and globular clusters (dense groups of old stars found in the centre of galaxies). We use these objects because we get to have an estimate of their distances, which is generally very hard to do in astronomy. Knowing the distance and the redshift of these galaxies (i.e. how fast they are moving away), which is relatively easy to get, astronomers can get an estimate of Hubble's constant. There are two groups of people finding two different values. Some believe the Hubble constant is on the order of 55 km/s/Mps, while others find something around 85 km/s/Mpc.
I cannot give you a firm answer on the value of the constant, because there are so many estimates, without much to decide which is the best. Recent analysis of Cepheid variables have given a value of 72+/-8 km/s/Mpc (you can see the large error bars put on that value!), but some people argue that it might be overestimated. You can go and check this website for a list of experiments and results for the value of the constant.
Update (May 2003) : new results
The satellite mission WMAP has produced in the past couple of months some new and exciting results! Among other things, it has enabled astronomers to calculate the value of the Hubble constant. They find a value of 71 km/s/Mpc, within 5% of error .
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